We use LinkedIn, Ryze for business networking but we forget to mention Twitter , a micro-blogging service, that appears in most of the blogs.This is how Wikipedia describes Twitter:
Twitter is a social networking and micro-blogging service that allows users to send “updates” (text-based posts, up to 140 characters long) via SMS, instant messaging, the Twitter website, or an application such as Twitterrific. Twitter was founded in October 2006 by San Francisco start-up company Obvious Corp.
Updates are displayed on the user’s profile page and also instantly delivered to other users who have signed up to receive them. The sender can restrict delivery to those in his or her circle of friends (delivery to everyone is the default). Users can receive updates via the Twitter website, instant messaging, SMS, RSS, or through an application.
Some of the benefits of Twitter is (obviously) networking, getting updates from friends,announcements of your blog deatils(and new articles on your blog) and so on…Noticed a very good presentation on “Twitter and the entire Web 2.0 phenomena revolving around it“and the interesting point is the title of the presentation “Twitter:Useless son of Web 2.0” , is it really useless? 
Enjoy Twitter(networking)!!!